"Error: Missing signature" result seems to not conform to saml-bindings-2.0

When my SP makes this authn redirect request (line breaks added for clarity):

https://mocksaml.com/api/saml/sso?
SAMLRequest=nJJBj9MwEIX%2FiuV7YieUJbE2kcpWiEoLVNvCgdvEmWwtYjt4JsDy61GbRSqXCu3R9nwz73neLYEfJ7Oe%2BRge8PuMxOKXHwOZ00Mj5xRMBHJkAngkw9bs1x%2FuTZlrA0SY2MUgL5DpOjOlyNHGUYrtppGuz2pti27o6je1fm2Hqi46WNXVCiscVjd9NUDRwQAFSvEFE7kYGlnmWoot0YzbQAyBG1nq8lWmy6woDsWNKVem1Hmhy69SbJDYBeAzeWSeyCjlo%2F12Epvb6BVMTp0OiihKsf5r6i4Gmj2mPaYfzuLnh%2FuFN0qN0cJ4jMS549%2BPuUdT6UovTcCSFLtnl29d6F14vP4l3VJE5v3hsMt2n%2FYH2Z7XYs4ek3gXkwe%2B3uR04%2FpsOJcaDOz4Sbb%2FodcjQw8Mt%2BpiZPsci4%2FgcbvZxdHZpxfI4ASBHAaWYj2O8eddQmBsJKcZpWqXkf%2BGr%2F0TAAD%2F%2Fw%3D%3D

&SigAlg=http%3A%2F%2Fwww.w3.org%2F2000%2F09%2Fxmldsig%23rsa-sha1

&Signature=auEkgD83piMUaxm%2BetHGmxDHpQV9b3t9CLxriYGdmklSH5ZH8aWku4zeFz8sEtZoAA6JiXLkEAIKbEQfee%2BsX70g%2FhVjPC9w%2BVTGjBbbJd98CEgtSvDWMB9AsfEtPw59kO5mux%2BcSuAXyfRberO96vcjF4X5WF27wA7A7qDT6RwkzK7V%2BQ0%2FesVDu1AGJkXNUJZv9EjZOtEnOymlPgLufpAlD5dPnR99Ktf3G1bJT7KDWi9V1TTizq5xr6rA5%2BocVnHEZN7ZPiCcGZfgtbjCJ0ZIkMpGG6ciZoPW00w00fXPRcdB%2BGIJuQT%2BXbiHYhzMHl3y7UMAZ7FVgkaRqmzJ%2BQ%3D%3D

Then the response page only shows ""Error: Missing signature".

FWIW I identified this section of code as the origin of that response

https://github.com/boxyhq/mock-saml/blob/b6f2e89ff6e9663e6b289e11c69722be2cbf37d7/utils/request.ts#L34-L40

From the saml-bindings-2.0 specification, section 3.4.4, it states

A query string parameter named SAMLEncoding is reserved to identify the encoding mechanism used. If
this parameter is omitted, then the value is assumed to be
urn:oasis:names:tc:SAML:2.0:bindings:URL-Encoding:DEFLATE

As such, DEFLATE is the mechanism in play, which then is discussed in section 3.4.4.1. Item 1 of XML serialization states:

Any signature on the SAML protocol message, including the ds:Signature XML element itself,
MUST be removed

which is what the SAML authentication library that I am uses does. That seems to be a mismatch with the expectation of the code referenced above, but I might be missing some broader context of the code.

Further in 3.4.4.1 the block that starts:

If the underlying SAML protocol message is signed with an XML signature [XMLSig], the URL-encoded
form of the message MUST be signed as follows:

You'll note that the URL I attempted, shown above, includes SigAlg and Signature, but they don't seem to be considered by the request processing.

Start with utils/request.ts lines 34-40 and reproduce the supplied redirect URL. Compare its handling of SigAlg and Signature with SAML Bindings 2.0 sections 3.4.4 and 3.4.4.1. Done means determining whether this valid signed request is processed according to the binding and documenting or correcting the missing-signature behavior.